NavList:

Bill wrote:

> In a deliberate set of sun off and on the arc this afternoon, I took the
> precaution of moving the drum at least 30' clockwise and then anticlockwise
> (bringing the reflected image down) instead of the usual 5-10 minutes
> twiddle.  In 12 observations each on and off the arc, it *seems* to have
> increased my sigma over the small twiddle method.

Someone may ask, "If repeatable, why do you think that is?"

One possibility is lateral play in the arm.  The arm and arc have a tongue
and groove arrangement.  Even with lubrication, there is slight lateral
play.  As the arc and worm gear teeth are slightly diagonal, turning the
drum one way will force the arm in, turning the other way will force the arm
out.  Again, as the teeth are diagonal, this lateral movement will result in
fore and aft movement of the arm as well once he teeth are set, changing
tangency by 0.1' to 0.3' minutes if I press the arm in then out.  If I read
Ken G. correctly this lateral movement is not uncommon in even top of the
line (cost wise) sextants.

The point here being that when I am moving quickly I do not take my fingers
off the drum and recheck.  Set tangency, record, and move on.  When I am
moving slowly and methodically I do remove my fingers, read the drum, then
recheck to see if there is a change in tangency.  In between quickly and
slowly, I bring the images near tangency, remove the fingers, look, tweak
and look.  From time to time I will see what was tangency become a slight
overlap, or a barely visible separation.  So I have to back off and do it
again.  I believe this is just the arm returning to a neutral position after
the pressure from the drum is removed.  Trivial, but a few tenths here and a
few tenths there in star-to- star, one star or sun IE, or lunars...

Bill


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